1. Technical Field
This disclosure relates to data transfer devices and more particularly, to a data transfer device which reduces delay between output data and a clock cycle.
2. Description of the Related Art
In digital devices, such as memory devices, data transfer to and from first in/first out devices (FIFOs) is controlled using input/output pointers. FIFOs are memory devices which output data in the same order as entered at the input. A relationship between the pointers which control the FIFO and a clock need to be well defined. It is particularly desirable for high frequency designs to maintain a substantially constant delay between the pointers and the clock.
Referring to FIG. 1, a conventional 1 out-of-8 decoder 10 is shown. Using binary counters for input and/or output pointer generation employs decoder 10 to output a bit representing a number input from binary counters. To decode a xe2x80x9c0xe2x80x9d inputs are Q0=0, Q1=0 and Q2=0. The pointer output for a FIFO-latch  less than 0 greater than  is xe2x80x9c0xe2x80x9d all others are one. Decoder must invert all xe2x80x9c0xe2x80x9ds to xe2x80x9c1xe2x80x9ds and NAND them using invertors 12 and NAND gate 14, respectively to output decoder FIFO latch  less than 0 greater than . For decoding a xe2x80x9c7xe2x80x9d the binary counter signals to decoder are Q0=1, Q1=1 and Q2 =1. The pointer output for a FIFO-latch  less than 7 greater than  is xe2x80x9c0xe2x80x9d all others are one. Decoder uses all xe2x80x9c1xe2x80x9ds without using invertors. NAND gate 16, NANDs the inputs at decoder for FIFO latch  less than 7 greater than  to provide an output of xe2x80x9c1xe2x80x9d. Other pointers for FIFO latches  less than 1 greater than  to  less than 6 greater than , have different inverter combinations in decoder to decode the binary counter inputs. timing diagram shows a clock pulse (CLK) as compared to the pointers for FIFO latches  less than 0 greater than  and  less than 7 greater than . The decoding of a xe2x80x9c0xe2x80x9d includes invertors 12 which introduce additional delay between the pointer from FIFO latch  less than 0 greater than  and CLK. The delay is represented by td+xcex4. The decoding of a xe2x80x9c7xe2x80x9d does not include invertors. The delay is represented by td. The additional delay of xcex4 is typically in the order of hundreds of pico seconds. This uncertainty in the pointer signals with respect to CLK is not desired.
Referring to FIG. 3, another timing diagram is shown for a prior art FIFO illustratively depicts a clock signal CLK and pointer signals PTR  less than 0 greater than , PTR  less than 1 greater than , and PTR  less than 7 greater than . Pointer signals each have a delay xcex4 ciated therewith and indicated in the timing diagram. As is apparent from FIG. 3, xcex40xe2x89xa0xcex41xe2x89xa0 . . . xcex47. This inequality between delay leads to differences in time for data output Dout as indicated by xcfx840, xcfx841 and xcfx847. xcfx840xe2x89xa0xcfx841xe2x89xa0 . . . xe2x89xa0xcfx847 which results in a skew problem.
Therefore, a need exists for pointer generation which includes pointers having a substantially same delay with respect to a clock. A further need exists for a method of providing FIFO memory with pointers with the substantially same delay with respect to the clock.
A pointer generation circuit, in accordance with the invention, includes a clock for providing a clock cycle, and a shift register with a plurality of latches for storing data bits. A first latch receives a flag bit upon a first clock cycle of the clock. A switch transfers the flag bit to the shift register on the first clock cycle. The switch connects a last latch to the first latch after the flag bit is transferred to the shift register. The flag bit is transferred to a next latch, wherein the next latch for the last latch is the first latch, at each consecutive clock cycle thereby generating pointer signals in accordance with the clock cycle and the data bits stored in the latches. The flag bit is transferred to a next latch, wherein the next latch for the last latch is the first latch, at each consecutive clock cycle thereby generating pointer signals in accordance with the clock cycle and the data bits stored in the latches.
Another pointer generation circuit in accordance with the invention includes a clock for providing a clock cycle. A shift register includes a plurality of latches for storing data bits. A first latch receives a flag bit upon a first clock cycle of the clock. A switch transfers the flag bit to the shift register on the first clock cycle. The switch connects a last latch to the first latch after the flag bit is transferred to the shift register. The flag bit being transferred to a next latch, wherein the next latch for the last latch is the first latch, at each consecutive clock cycle thereby generating output signals in accordance with the clock cycle and the data bits stored in the latches. A pulse generation circuit is included for receiving the output signals and generating a pointer signal having a predetermined pulse width.
In alternate embodiments, the flag bit is preferably a digital xe2x80x9c1xe2x80x9d and the latches other than the latch including the flag bit are digitalxe2x80x9c0xe2x80x9ds. The shift register may include eight latches. The latches of the shift register may output to a first in/first out (FIFO) memory device. Each clock cycle may include a first signal edge and the latches preferably output a pointer signal such that a time delay for the pointer signal from each latch is substantially equal relative to the first signal edge of a corresponding clock cycle. The circuit may be included on an integrated circuit chip.
A method for pointer generation for first in/first out memories includes the steps of providing a pointer generation circuit including a clock for providing a clock cycle, a shift register including a plurality of latches for storing data bits, a first latch for receiving a flag bit upon a first clock cycle of the clock, initializing the latches to a data bit value, transferring the flag bit to the first latch, connecting the last latch to the first latch, transferring the flag bit to a next latch, wherein the next latch for the last latch is the first latch, corresponding to each clock cycle and generating pointer signals in accordance with the clock cycle by outputting the data bits stored in the latches.
In other methods, the flag bit is preferably a digital xe2x80x9c1xe2x80x9d and the data bit value is a digital xe2x80x9c0xe2x80x9d. The shift register may include eight latches. The step of generating pointer signals may include the step of generating pointer signals in accordance with the clock cycle by outputting the data bits stored in the latches to a pulse generating circuit. The method may further include the step of generating pulses of a predetermined pulse width by the pulse generating circuit. Each clock cycle may include a first signal edge, and the method may further include the step of outputting a pointer signal from the latches such that a time delay for the pointer signal from each latch is substantially equal relative to the first signal edge of a corresponding clock cycle. The pointer generation circuit preferably includes a switch and the method may further include the steps of transferring the flag bit to the shift register on the first clock cycle through the switch, connecting a last latch to the first latch through the switch after the flag bit is transferred to the shift register.
These and other objects, features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings.